Transformation Of Intragranular Nucleation Ferrite And Microstructural Nano-structuring Of V Microalloyed Steel

In this paper, the the research work related to transformation mechanisms of intragranular nucleation ferrite in V mciroalloyed steel and its application in strengthening and toughening steel as well as microstructural nano-structuring was concucted. Firstly, the fundamental theory about the effect of nano-scale VN precipitates on intragranular ferrite nucleation was systematically studied, and the theory of VN precipitates stimulating intragranular ferrite nucleation was applied to the control of microstructural homogeneity in ultra-thick plate and the control process of micro structures, strength, and toughness during welding thermal cycle. The microstructural homogenenity in thickness direction was improved, and the micro structures and toughness of welding joints were enhanced. Secondly, the stimulation effect of VN precipitates on ferrite transformation was applied to the fabrication of nano-grained steel. The formation mechanisms of nano-grained microstructure in V-N steel and the superplasticity of nano-grained steel were studied. The main study work and the innovative achievements are as follows:(1) The ferritic nucleation behavior of Ti-V steel and single Ti steel was compared. It was concluded that the potency of intragranular ferrite nucleation was enhanced by V modifing the surface of the Ti carbonitrides. The grain was refined, and the strength and toughness were improved. The VN precipitation behavior in austenite of V-N microalloyed steel and its effect on intragranular ferrite nucleation were studied by thermal simulation experimental technology. The results showed that-10-20nm VN precipitates were formed during 850℃ isothermal holding process, and played significant effect on stimulating intragranular ferrite nucleation during isothermal holding process of medium temperature transformation region at the condition of high supercooling.(2) The thermo-mechanical control process (TMCP) experiment in laboratory was conducted on V-N microalloyed steel, the micro structures, V(C,N) precipitation behavior, and strength and toughness properties of different thickness in the steel plate of 80mm thickness were studied. The results showed that the homogeneous and good microstructural distribution in the thickness direction of the steel plate of 80mm thickness was obtained by taking advantage of V(C,N) precipitates in austenite stimulating intragranular ferrite nucleation. The microstructures of both 1/4 and 1/2 thickness consisted of polygonal ferrite, acicular ferrite, and a small amount of pearlite. The comprehensive mechanical properties were excellent.(3) The microstructural evolution and properties of strength and toughness in simulated coarse grained heat-affected zone (CGHAZ) and intercritically reheated coarse-grained heat-affected zone (ICRCG HAZ) of high strength V-N mciroalloyed steel during welding thermal cycle were studied by taking advantage of thermal simulation experimental technology. The results showed that the nano-scale VN precipitates formed in the austenite of simulated CGHAZ and stimulated intragranular acicular ferrite nucleation at the medium heat input, and excellent low temperature impact toughness was obtained. The large size and necklace type of M/A constituent along the prior austenite grain boundaries (PAGB) was not observable in the ICRCG HAZ. The ～20-30nm V(C,N) precipitates were the nucleation sites of the fine ferrite along PAGB, and the formation of the granular bainite with low misorientation boundaries was substituted. The crack propagation energy was increased. The crack initiation energy was enhanced because of the ultra refinement of martensite/austenite (M/A) constituent.(4) The warm rolling experiment was conducted on V-N-Cr microalloyed steel, and the microstructures and mechanical properties were studied. The results showed that the grain could be refined to-300-400nm in the steel plate of 5.5mm thickness by combining the mechanisms of dynamic transformation, intragranular nucleation ferrite, and dynamic recrystallization. The ferrite-martensite micro structure with nanoscale features obtained by warm rolling and quenching was of excellent tensile properties including yield strength, tensile strength, and elongation to fracture of 745MPa,935MPa, and 19.5%, respectively.(5) The martensite cold rolling and annealing experiments were conducted on V microalloyed steel. The effect of annealing temperature, annealing time, deformation amount, and N content on the microstructural evolution and grain nanostructuring was studied. The results showed the 200nm equiaxial ferrite and 30-50nm cementite were obtained in the cold rolled high N steel of 0.9mm thickness during annealing at 550℃ for 5min, and the high thermal stability during long time isothermal holding at 500℃.(6) The microstructural homogeneity was improved as increasing the deformation amount of martensite cold rolling, while the effect on microstructural stability was weak at high temperature. The addition of high N stimulated the VN precipitation, and the nucleation of recrystallization ferrite was accelerated. The room temperature tension was controlled by dislocation movement, thus the room temperature tensile properties of ultra-fine grained steel is good. The high temperature tension depended on the mechanisms of grain boundaries slide and rotation. The low temperature superplasticity was obtained in the nano grianed structure comprised of 200nm equiaxial ferrite grains and 30-50nm cementite during tension at 500℃ of lower than 0.5 melting temperature (Tm).